Incineration consists of burning the dried solids to reduce the organic residuals to an ash that can be disposed or reused. Incinerators often include heat recovery features. Undigested sludge solids have significant fuel value as a result of their high organic content. However, the water content must be reduced to a large amount by dewatering or drying for taking advantage of the fuel potential of the bio solids. For this reason, pressure filtration dewatering equipment is used to obtain bio solids, which are sufficiently dry to burn without continual reliance on auxiliary fuels. In some cities, bio solids are mixed with refuse or refuse derived fuel prior to burning. Generally, waste heat is recovered to provide the greatest amount of energy efficiency. (EPA 2004)